1. Field of the Invention
The present invention generally relates to an on-vehicle electronic apparatus such as a car stereo apparatus, and more particularly to an interface between an operation and display unit and a main body unit in such an on-vehicle electronic apparatus.
2. Description of the Related Art
There are two types of car stereo apparatuses. Namely, one type is fixedly mounted in a cockpit of a vehicle. The other type is detachably mounted on the vehicle i.e., installed to the vehicle in such a manner to be easily detached and attached by the owner (user) of the vehicle. The reason why it is detachably mounted is to detach the electronic apparatus as a crime prevention when the owner is away from his vehicle.
As a method of detaching and attaching this type of car stereo apparatus, there is a partially detachable method of detaching only the major portion of the car stereo apparatus, and a whole detachable method of detaching the whole car stereo apparatus from the cockpit. Hereinbelow, the partially detachable method will be explained.
In the partially detachable method, the operation panel is detachably mounted on the front of the cabinet. After the operation panel which is the major portion of the car stereo apparatus is detached, it cannot function as a car stereo apparatus anymore. Thus, the crime prevention is enabled.
The main body unit is included in the cabinet, and is fixedly mounted with respect to the vehicle body. The main body unit may be provided with a main body side microcomputer, an oscillator, audio equipments and so on. The main body side microcomputer controls transmission and reception of various signals. The oscillator generates a clock. The audio equipments may be a tape deck circuit and a tuning circuit for achieving a synchronization of the tuner, for example.
On the other hand, the operation panel may be provided with a LCD (Liquid Crystal Display) microcomputer, a LCD device, a key input unit disposed at the front of the operation panel, a light receiving unit for remote control by infrared ray and so on. The LCD microcomputer receives the input from the key input unit and the light receiving unit for remote control, transmits the key input to the main body side microcomputer, receives the display data transmitted from the main body side microcomputer, and drives an internal LCD driver to display it on the LCD device.
The LCD microcomputer and the main body side microcomputer are connected to each other by five transmission lines i.e. a clock line, a data line, a chip select line, a busy line and a key line.
The clock line is a line for transmitting the clock, which is generated by the oscillator, from the main body side microcomputer to the LCD microcomputer. The data line is a line for transmitting the data, which indicates the display content, from the main body side microcomputer to the LCD microcomputer. The chip select line is a line for transmitting a chip select control signal, which controls the chip selection in the LCD microcomputer, from the main body side microcomputer to the LCD microcomputer. The busy line is a line for transmitting a signal, which indicates the LCD microcomputer is in a status to prohibit the display data reception of the LCD microcomputer, from the LCD microcomputer to the main body side microcomputer. The key line is a line for transmitting the key data, which is inputted from the key input unit, from the LCD microcomputer to the main body side microcomputer.
In this manner, there are required at least five transmission lines in case of the above mentioned car stereo apparatus, in which the operation panel is detachable from the main body unit.
However, in this kind of car stereo apparatus, metal contacts are required to transmit and receive the electrical signals between the operation panel and the main body unit. Thus, noises such as a pulse are generated by the vehicle vibration and by the change in conditions between contact and non-contact at the metal contacts when the operation panel is detached and attached. If there is generated such a noise during the signal transmission, the transmitted signal is destroyed.
For example, the display data, which are transmitted from the main body side microcomputer to the LCD microcomputer on the operation panel side through the clock line, the data line and the chip selection line, are transmitted by a clock type serial transmission. Accordingly, when the noise (e.g. a small pulse) enters the clock line, since the LCD microcomputer erroneously counts this noise as one clock, the LCD microcomputer receives erroneous data as a result. On the other hand, since the transmission method of the key data is the PWM (Pulse Width Modulation) method, when the noise is generated in this case, the erroneous count is generated and erroneous data is also taken in.
Especially, in case of the car stereo apparatus in which the operation panel can be detached from the main body unit, since there are required at least five transmission lines, the possibility to generate the noise due to the contact and non-contact at the metal contacts is high.